Professor Peter Openshaw, director of the Centre for Respiratory Infection at Imperial College London told Channel 4 News it is highly unusual to see more than 100 new cases in three weeks and he is very concerned.

He said: “This is clearly one of the most pathogenic viruses that we’ve seen, so far 20 per cent (of sufferers) have died, but only 20 per cent have recovered, so that leaves 60 per cent who are in between and may die or may recover, so in terms of severity it ranks amongst the most lethal viruses that we’ve had yet to face.”

‘No natural immunity’ to bird flu

H7N9 has infected people of all age groups from 2-years-old to 81 years old suggesting that humans have no natural immunity to it. Another challenge scientists face is that infected poultry do not present signs of sickness.

Professor Wendy Barclay, chair of influenza and virology at Imperial College London said this type of virus emerges every decade or so with the ability to cross into people and there is real worry about the virus spreading via migratory birds.

Speaking to Channel 4 News, she said: “Certainly if we look to the H5N1 virus which is now spread all the way from where it began in South East Asia to Africa and the edges of Europe, we can see that migratory birds played a big role in the spread of that virus, and there’s no reason why they shouldn’t do so with this new H7N9.”

Currently the virus has made its home in chickens, and has affected people who have had close contact with the birds, often at live markets. But there have also been some cases of people who have no had contact with chickens also falling ill.

Since 2003 H5N1 has led to 628 confirmed cases in 15 countries and 374 deaths.

So far, H7N9 does not have the ability to replicate itself within a human cell. But if the virus were to become fully adapted to human hosts it could result in a serious worldwide pandemic claiming millions of lives.

Attempts to develop vaccine

Dr John McCauley, an expert in host specificity of influenza viruses at the Medical Research Council’s National Institute for Medical Research, is heading up a team trying to develop a possible vaccine.

He said: “For us, what we’re trying to do is to infect the ferrets and get antibody from the ferrets, and so what we can do with that anitbody is we can actually check whether or not any candidate vaccine that is to be developed would be suitable – that is, antigenically similar to the virus that’s out there, then we can say this virus is suitable for a vaccine development.”

According to experts the virus is said to be sensitive to antiviral drugs such as Tamiflu.

But Dr John Watson, head of respiratory diseases at Public Health England, the body that has taken over from the Health Protection Agency, said it was important not to assume there was no threat to the UK.

“There are still large stocks of Tamiflu in the UK following the swine flu pandemic of 2009-10. But currently there is no readily available vaccine against the strain H7N9.